| Title |
Investigators | Department | Objectives | Approach Keywords | Progress Reports | Impact Statements | Publications | |
Project * COL00175 | |
| Title | *Integrating multiple biological control agents for Dalmatian toadflax and diffuse knapweed |
| Investigator(s) | Norton, AP; Hufbauer, RA; |
| Department | Bioagricultural Sciences and Pest Mgmt. |
| Objectives | Project 1: Experimentally determine the effects of insects and mowing together and separately on Dalmatian toadflax. Project 2: Experimentally determine the effects of above- and below- ground herbivory by insect biocontrol agents on diffuse knapweed. |
| Approach | Project 1: Two well-established biological control agents of Dalmatian toadflax are Brachypterolus pulicarius flower-feeding beetles and Gymnetron antirrhini fruit-feeding weevils. While data exist on the impact of the beetles on yellow toadflax (L. vulgaris), there are no published data on the utility of these beetles for controlling Dalmatian toadflax. Grazing by goats is a tool that has worked well with some perennial weeds and may be effective against Dalmatian toadflax. Before we evaluate the potential for combining goat grazing and insect biocontrol agents we need a better understanding of the interaction between early season plant defoliation and the performance and impact of flower- and fruit-feeding insects. We will experimentally examine the interaction between early season plant defoliation and beetle performance and impact in replicated small plots. Four treatments (defoliation alone, defoliation and beetles, beetles alone, no beetles and no defoliation) will each be replicated 8 times in 20 m x 20 m plots. Three times each year we will measure the abundance, height and surface area of Dalmation toadflax plants in each plot. Beetle densities will be estimated from beat samples of the foliage and flowers and the number of viable seeds produced by the plants will be estimated from collections of the seed capsules as they ripen. These seeds will be returned to the lab to measure germination rates. Project 2: Three biological control agents that attack the roots of diffuse knapweed and three that attack the seed heads are all established at plots in Boulder County. We will establish four treatments within these plots: attack by root and seed-feeding insects (no pesticide control), attack by root feeding insects only (above-ground pesticide only), attack by seed feeding insects only (below-ground pesticide only), and no herbivory (above- and below- ground pesticides). Ten 3 m X 3 m replicates of each treatment will be established and maintained for two years. Three times each year we will measure population levels of the biological control agents in each of these plots through collection of a random sample of 10 plants. Plant performance data will be collected as well. From each plot we will determine the number of living C. diffusa, the height of the plants and the proportion of the plot covered. Focal plants taken for estimates of insect abundance will be returned to the lab and the average weight of each plant will be determined. With these data we will test the hypothesis that the impact of multiple herbivores on plant performance is additive. Alternative hypotheses are that the impact of root and seed feeding herbivores is reduced by the presence of the other feeding guild, or that the impact is enhanced when both types of agent are acting together. |
| Keywords | Biological Control, Impacts of multiple herbivores, Herbivore-herbivore competition |
| Progress Reports | |
| 2002 | This project seeks to determine if we can integrate to different methods of weed management: intensive grazing by goats and biological control. Both strategies have been used separately, and this project seeks to determine if: A) If grazing or arthropod biological control can reduce the density and spread of Dalmatian toadflax; B) If goat grazing and arthropod biological control are compatible; and C) If the impact of these two management techniques is greater than would be expected given their individual impacts on the plant. In the summer of 2002, we conducted a trial to examine the impact of goat grazing on Dalmatian toadflax and the arthropod biological control agent, Brachypterolus pulicarius. We used 8 50m x 50m plots located at the Nature Conservancy Phantom Canyon preserve. This preserve is located ca. 30 miles north of Fort Collins, CO. Research plots were situated in short grass prairie at an elevation of ca. 6,500 feet. We used a nested design to examine the impact of goat grazing on toadflax stem density, flowering phenology and seed production, and the density of the seed feeding beetle, B. pulicarius. Four plots were intensively grazed by goats (100 goats per plot for 12 hours) and 4 were left un-grazed. Within these larger grazing manipulations, we randomly selected four 4 m x 4 m subplots within each plot for data collection. Goats readily fed upon toadflax in these plots. Averaged over the four grazed plots, more than 50% of all toadflax stems were fed upon, compared to less than 10% in the un-grazed plots (most likely from mule deer or antelope resident in the preserve). Although a high percentage of Dalmatian toadflax stems were fed upon, this did not result in a significant reduction in the percentage of stems producing flowers. 2002 was an extreme drought year in Colorado, and many native plants at this site failed to flower. In our plots, flowering by Dalmatian toadflax was rare, with only 15% of stems producing any flowers at the peak of the first bloom period. Averaged over the entire season there was no indication that goats reduced the percentage of stems flowering, though there is a (non-significant) pattern of goat feeding resulting in a shift of flowering to later in the season. Averaged over the season, the number of B. pulicarius per plant stem was correlated with the percentage of stems flowering. This is not surprising as both adults and larvae spend the majority of their time inside of toadflax flowers. Interestingly, the relationship between flowering stems and increased beetle densities is much stronger for un-grazed than for grazed plots. Although grazed and un-grazed plots had similar proportions of stems flowering throughout the season, un-grazed plots with a high proportion of flowering stems harbored more beetles than grazed plots with similar proportions of flowering stems. This was the first year of a multi-year trial and we still need to collected data in the spring of 2003 to fully evaluate the impact of grazing on toadflax. These preliminary data indicate that goats do feed upon the weed, and there is the possibility of a negative interaction between goat grazing and biocontrol agent populations. |
| 2003 | This project seeks to determine if we can integrate different methods of weed management: intensive grazing by goats and biological control. Both strategies have been used separately, and this project seeks to determine if: A) If grazing or arthropod biological control can reduce the density and spread of Dalmatian toadflax; B) If goat grazing and arthropod biological control are compatible; and C) If the impact of these two management techniques is greater than would be expected given their individual impacts on the plant. We used 8 50m x 50m plots located at the Nature Conservancy Phantom Canyon preserve. This preserve is located ca. 30 miles north of Fort Collins, CO. Research plots were situated in short grass prairie at an elevation of ca. 6,500 feet. In the spring of 2002 and again in 2003, four of these plots were subjected to intensive grazing (100 goats per plot for 12 (2002) or 24 (2003) hours, and four were left as un-grazed controls. We subsequently measured toadflax and native plant abundance and density. In both years, a substantial number of toadflax stems were grazed by the goats. In 2002 over 50% of toadflax stems were grazed and in 2003 over 90% of the stems were removed. However, toadflax plants re-grew rapidly. Six weeks following intensive grazing there were no significant differences in toadflax stem densities between the plots in either year. In 2003 after being subjected to two years of goat grazing, Dalmatian toadflax produced significantly fewer flowers and fruits than non-grazed plants. Grazed plants produced on average 8 fruits per stem in contrast to over 100 fruit for un-grazed stems. We have yet to see a reduction in stand density due to intensive grazing, but grazing has dramatically reduced seed set of the weed. So far, it does not appear that intensive grazing will reduce toadflax densities at this site. These plots will be subjected to further grazing in 2004 and we will continue to evaluate the cumulative impact of intensive grazing on Dalmatian toadflax stand density and seed set. |
| 2004 | This project seeks to determine if intensive grazing by goats will reduce Dalmatian toadflax abundance or cover. We have subjected large replicated blocks of toadflax infested native short grass steppe to intensive grazing for three years. In the spring of 2002 we used 100 goats per acre for 12 hours, in 2003 and in 2004 we used 120 goats for 24 hours. This grazing intensity was sufficient to remove most of the above ground biomass of the weed. After grazing we measured toadflax cover, abundance, flower and seed set and plant community composition. In 2004, Dalmatian toadflax re-grew rapidly, and by the end of the season there were the same number of toadflax stems in grazed and control plots. After three years of intensive grazing we have yet to see any impact on toadflax stand density or cover at this site. Grazing did delay the start of flowering by approximately one month, and by the end of the season grazed plots had produced only about 2/3 the number of flowers and fruit as un-grazed controls. At this site, the impact of goat grazing on Dalmatian toadflax appears to be limited to a moderate reduction in seed set. |
| 2005 | Progress on Project 2. Diffuse knapweed is an exotic, noxious weed common in Colorado. Several arthropod biological control agents have been introduced against this weed. This study examines how plant competition and either root feeding or foliage feeding biological controls interact to determine diffuse knapweed biomass and competitive ability. Methods: Diffuse knapweed was grown in the presence or absence of either of two competitors: blue grama (Bouteloua gracilis) or fringed sage (Artemisia frigida). Crossed with these three competition treatments diffuse knapweed plants were caged with either the foliage feeding weevil Larinus minutus or the root feeder Cyphocleonus achates, or neither. An additional set of pots held only blue grama or fringed sage. All plants were started from seed, and were grown in locally collected field soil. Results: Diffuse knapweed above ground biomass was always less when grown with either of the competitor species than when grown alone. Averaged over all competition treatments, both L. minutus and C. achataes reduced knapweed biomass by a similar amount (~ 10%). However, both of these biological control agents had a much greater impact on knapweed in the fringed sage treatment than in either the blue grama or control treatments. Blue grama above ground biomass was greater when grown in the absence of competition. When grown in the presence of diffuse knapweed there was a significant increase in blue grama biomass when either biological control agent fed on knapweed. Similarly, fringed sage plants were always smaller when grown in the presence of diffuse knapweed, but the effect of competition was reduced (though not significantly so) when knapweed was attacked by the biological control agents. These results demonstrate that (1) competition increases the impact of these biological control agents on diffuse knapweed, and (2) that there is some evidence that sub-lethal attack by either of these biological control agents reduces knapweed competitive ability and enhances native plant growth. |
| 2006 | Diffuse knapweed is an exotic, noxious weed common in Colorado. This species has been reported to release a phytotoxic chemical from its roots, 8-hydroxyquinoline. Production of this herbicidal compound may help diffuse knapweed out-compete native vegetation, thus contributing to the spread of this weed. Several arthropod biological control agents have been introduced against diffuse knapweed, but other studies have suggested that herbivory may increase the production of similar toxic root exudates in other plant species. This study extends previous work and examines if herbivory increases diffuse knapweed's production of the toxic allelochemical 8-hydroxyquinoline. Methods: Diffuse knapweed was grown in the presence or absence of either of two competitors: blue grama (Bouteloua gracilis) or fringed sage (Artemisia frigida). Crossed with these three competition treatments diffuse knapweed plants were caged with either the foliage feeding weevil Larinus minutus or the root feeder Cyphocleonus achates, or neither. An additional set of pots held only blue grama or fringed sage. All plants were started from seed, and were grown in locally collected field soil. After 4 weeks of feeding by the biological control agents, we measured the concentration of 8-hydroxyquinoline in the soil. Results: Feeding by either biological control agent significantly reduced diffuse knapweed biomass when the plant was grown with competing vegetation. However we did not detect 8-hydroxyquinoline in any of the experimental soils, down to a detection threshold of 1 ppm. We also examined soils from 12 different natural knapweed populations and were unable to detect the chemical in these as well. Previous publications have reported the compound to commonly reach concentrations of more than 300 ppm . Our results suggest that diffuse knapweed is unlikely to produce 8-hydroxyquinoline and that previously published reports by others should be re-examined. |
| Impact | |
| 2002 | A synergism between grazing and biocontrol could greatly improve our ability to manage this invasive plant species in range and parkland. In contrast, a negative interaction between the control agents would indicate that a sequential use strategy would be more successful than a simultaneous use strategy. Determining the nature of the interaction between these control strategies will increase the efficiency of weed management efforts. |
| 2003 | A synergism between grazing and biocontrol could greatly improve our ability to manage this invasive plant species in range and parkland. In contrast, a negative interaction between the control agents would indicate that a sequential use strategy would be more successful than a simultaneous use strategy. Determining the nature of the interaction between these control strategies will increase the efficiency of weed management efforts. |
| 2004 | Land managers have used a variety of strategies to manage invasive weeds. This research evaluates one strategy, intensive grazing by goats, that has been commonly used. This research will provide important information to land managers about the results of this management tactic on weed abundance and plant community composition. |
| 2005 | Impact. Invasive weeds have enormous economic and environmental impacts. Biological control of noxious weeds may offer the most efficient and environmentally sound methods for exotic weed management. These results demonstrate that the presence of competing vegetation enhances biological control agent impact and that biological control agents may increase native plant competitive ability. Thus strategies that enhance the comptitive ability of desireable plant speices sould also enhance the efficacy of these biological control agents. |
| 2006 | Invasive weeds have enormous economic and environmental impacts. Biological control of noxious weeds may offer the most efficient and environmentally sound methods for exotic weed management. These results demonstrate that the presence of competing vegetation enhances biological control agent impact and that biological control agents may increase native plant competitive ability. |
| Publications | |
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